A typical wind turbine essentially comprises a tower and a rotor having a plurality of blades, e.g. two or three. During operation of the wind turbine, the rotor and its associated blades rotate e.g. as a function of the instantaneous wind strength. Mounted on the tower is a nacelle containing, among other things, a bearing for supporting the rotor and a generator for converting the corresponding rotational energy into electric power. Systems of this kind are well-known.
The sizes of such systems are continually increasing. For example, blades with a length in the order of 50 m or more are used for modern offshore wind turbines delivering nominal outputs in the single-digit megawatt range. The larger the structural dimensions, the greater the mechanical loads on blade, rotor, bearing, tower, and ultimately also on the foundations in which the tower is embedded.
For dimensioning the blades and for adjusting and controlling the wind turbine during operation, precise information is required concerning the dynamic states in particular of the blades. For example, blade deflection must be monitored in order to rule out the possibility of the blades striking the tower of the wind turbine as they rotate. The probability of a blade being bent to the extent that such a strike can occur naturally increases with blade length and with the wind force instantaneously acting on the blade.
The wind turbines are generally oriented such that the wind direction parallels as far as possible the axis of rotation. The blades are consequently subjected to a force which pushes the rotating blades in the direction of the tower, the blade tip being deflected the farthest from a normal position in which the blade is not deformed, i.e. bent, that is to say in particular in situations in which there is no wind load on the blade.
For example, a control wire method can be used to detect any deflection of the blade by monitoring the length of a control cable stretched between the blade tip and another point which is ideally close to the axis of rotation. The problem with this method, in the specific case of the wind turbine, is the mounting of the control cable because of the rotating parts.
Strain gages can also be used which must be fixed to the surface of the blade and which are deformed correspondingly to the deflection of the blade. The output signal of the strain gages is then indicative of the deflection. The problem here is the high installation overhead and the susceptibility to lightning strikes, for example.
Another alternative are optical methods, such as laser distance measurement or analysis of camera images. Laser distance measurement is problematic in that mounting for an optimum measuring position is very complex: If a measuring position on the static part of the system is required, 360° monitoring of blade deflection, i.e. monitoring over the entire circumference swept by the blade tip, can only be implemented at high cost and with great complexity. Although camera optical systems mounted close to the rotor hub provide very accurate information about the dynamic behavior of the blade, a high level of overhead is required for the signal or, as the case may be, image processing. Moreover, the camera optical systems in particular cannot operate in all weather conditions.
A device for monitoring a wind power installation for possible blade-tower strikes is described in DE 20 2007 001 136 U1, for example. There, a distance sensor for contactless measurement of the distance of the blade from a predefined point on the wind power unit is installed thereon. If a critical clearance is not achieved, a strike warning is output. A radio-based monitoring system is described in JP 2008303882 AA.